1. Field of the Invention
The present invention relates to an optical disc apparatus, an information recording method, and an information reproduction method and is suitably applied to an optical disc apparatus that records a hologram on an optical disc and reproduces the hologram, for example.
2. Description of the Related Art
Known optical disc apparatuses that reproduce information by irradiating light beams on optical discs, such as a CD (compact disc), a DVD (digital versatile disc) and a Blu-ray disc (registered trademark; referred to as “BD”) and reading reflected light are widespread.
Moreover, in the known optical disc apparatuses, recording of information is performed by irradiating light beams on the optical discs and changing a local reflectance of the optical discs.
Regarding the optical discs, the size of a light spot formed on a corresponding optical disc is approximately given by λ/NA (λ: wavelength of a light beam, NA: numerical aperture), and it is known that the resolution is also proportional to the value. For example, details of a BD that can record about 25 GB data on an optical disc having a diameter of 120 mm are disclosed in Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys., 39, 756(2000) (Non-patent Document 1).
Various kinds of contents, such as music contents and image contents, or various kinds of information, such as various data for computers, are recorded on an optical disc. In particular, since an increase in the number of contents recorded in one optical disc is recently requested as the amount of information increases due to a high-resolution image or high-quality sound, the optical disc is requested to have a larger capacity than before.
For this reason, a technique of increasing the recording capacity in one optical disc by causing recording layers to overlap within one optical disc is proposed (for example, refer to I. Ichimura et al, Technical Digest of ISOM' 04, pp 52, Oct. 11-15, 2005, Jeju Korea (Non-patent Document 2)).
On the other hand, an optical disc apparatus using a hologram is also proposed as a technique of recording information on an optical disc (for example, refer to R. R. McLeod et al., “Micro holographic multilayer optical disc data storage,” Appl. Opt., Vol. 44, 2005, pp 3197 (Non-patent Document 3)).
For example, as shown in FIG. 1, an optical disc apparatus 1 is configured such that light beams from an optical head 7 are condensed into an optical disc 8, which is formed of a photopolymer whose refractive index changes with the intensity of irradiated light and then light beam from the opposite direction is condensed once again on the same focus position by using a reflection unit 9 provided in a bottom surface side (lower side in FIG. 1) of the optical disc 8.
The optical disc apparatus 1 makes a laser 2 emit a light beam, which is a laser light, modulates the light wave by using an acoustooptic modulator 3, and converts the modulated light wave into parallel light by using a collimator lens 4. Then, the light beams are transmitted through a polarization beam splitter 5, are converted into circularly polarized light from linearly polarized light by a quarter wavelength plate 6, and are then incident on the optical head 7.
The optical head 7 is configured to be able to record and reproduce information. The optical head 7 makes light beams reflected by a mirror 7A and condensed by an objective lens 7B so as to be irradiated onto the optical disc 8 which is rotating by a spindle motor (not shown).
At this time, light beams are condensed within the optical disc 8 and are then reflected by the reflection unit 9 provided at the bottom surface side of the optical disc 8, such that the light beams are condensed onto the same focus within the optical disc 8 from the bottom surface side of the optical disc 8. In addition, the reflection unit 9 is configured to include a condensing lens 9A, a shutter 9B, a condensing lens 9C, and a reflection mirror 9D.
As a result, as shown in FIG. 2A, a standing wave is generated on the focus position of a light beam and a recording mark RM that is a hologram, which has a small light spot size and has an overall shape obtained by attaching two cones such that vortexes thereof are made contact with each other, is formed. In this way, the recording mark RM is recorded as information.
When the optical disc apparatus 1 records a plurality of recording marks RM within the optical disc 8, the optical disc apparatus 1 can form one mark recording layer by rotating the optical disc 8 and disposing each recording mark RM along a track having a shape of a concentric circle or a spiral and can record each recording mark RM by adjusting the focus position of a light beam such that a plurality of mark recording layers overlap.
Thus, the optical disc 8 has a multi-layer structure including a plurality of mark recording layers thereinside. For example, as shown in FIG. 2B, in the optical disc 8, a distance (mark pitch) p1 between recording marks RM is 1.5 μm, a distance (track pitch) p2 between trucks is 2 μm, and a distance p3 between layers is 22.5 μm.
In addition, in the case of reproducing information from the disc 8 on which the recording mark RM is recorded, the optical disc apparatus 1 closes the shutter 9B of the reflection unit 9 such that a light beam is not irradiated from the rear surface side of the optical disc 8.
At this time, the optical disc apparatus 1 causes the optical head 7 to irradiate light beams onto the recording mark RM in the optical disc 8 and causes reproduction light beams generated from the recording mark RM to be incident on the optical head 7. The reproduction light beams are converted into linearly polarized light from circularly polarized light by the quarter wavelength plate 6 and are reflected by the polarization beam splitter 5. Then, the reproduction light beams are condensed by the condensing lens 10 and are irradiated onto a photodetector 12 through a pinhole plate 11.
At this time, the optical disc apparatus 1 detects the optical amount of reproduction light beams using photodetector 12 and reproduces information based on a result of the detection.